This invention is concerned with glue dispensing apparatus, and more specifically the invention relates to a glue head for delivering glue under pressure to a substrate such as advancing paper or other container material carried on a conveyor.
In the paper handling industry, especially those operations involving application of glues or other liquids to paper, cardboard, corrugated paper or other substrate materials, wherein those substrate materials are moved through one or more machines for various operations, it is important that the glue or other liquid be accurately positioned. For the application of glue to such substrates, normally a series of glue heads are mounted on a machine at strategic locations, for applying lines, dots or stitches of glue on tabs or flaps later to be folded and adhered to another portion of the substrate. In addition to the goal of accuracy in such glue application, another important goal is speed of the operation. Thus, the machine is typically set to convey the substrate as quickly as possible while still obtaining accurate results.
Liquid glue is fed to the glue heads under pressure, each glue head having a valve that is typically closed by a spring and opened by a solenoid or other electromagnetic device on receipt of an electrical pulse. The pulses or signals are sent by some form of controller, such as the computer controlled device and process disclosed in U.S. Pat. No. 5,479,352, assigned to the assignee of the present invention.
As disclosed in the referenced patent, there is always a lag period or "compensation time" associated with the dispensing of glue from each head, both an "ON" compensation time and an "OFF" compensation time. This is the duration of the lag between the sending of the signal for the valve to open or close, and the actual beginning or cessation of deposition of glue onto the substrate. Typically, "OFF" comp times have been longer than "ON" comp times for a given glue head.
In addition, when the speed of the operation is increased, as discussed above, this means a greater flow of glue must be effected from each head over time, i.e. more glue volume per second, in order to keep the glue volume per inch of substrate the same. To accomplish this, glue pressure must be increased. A problem arises with prior glue heads, in that when pressure is increased, this changes the "ON" and "OFF" comp times. For example, in a glue head with a valve essentially comprising a needle valve, ball, plunger or plate which closes by moving the ball or other device in the direction of glue flow toward a seat, thus closing the glue flow conduit, the glue pressure helps close and hold close the valve, so that any increase in glue pressure requires a greater pulling force to open the valve. This increases the "ON" comp time. Nozzle velocity is also a factor in changes of speed and pressure.
At higher and higher speeds, there is a need for shorter and shorter actual glue delivery durations. If the "OFF" comp time exceeds the "ON" comp time, a situation can arise in which the glue head "OFF" impulse would have to appear before the "ON" impulse to create a dot or short line of glue, making such a pattern impossible.
Accurate setup of an operation therefore becomes rather complicated, particularly with changes of speed. The above U.S. Pat. No. 5,479,352 addresses one of the problems involved with speed variations, but not problems generated by inconsistent action of the glue head itself.
An associated problem with contemporary glue heads is the large power consumption required for the solenoid in each head, increasing cost but more importantly, generating considerable heat in the glue head. The reason for the large power consumption is either a heavy spring force that must be overcome by the solenoid for each opening of the valve, or a strong tendency of the valve closure device to close the valve due to the geometry of the valve and the effect of glue pressure, as explained above. The high temperature expands components and adversely affects operation of the glue head, reducing accuracy in placement of the lines, dots or stitches of glue on the substrate. Such high temperature (often too hot to touch) of the glue heads often causes leakage, primarily because of change of dimensions of components and thus pressure on an O-ring seal or valve seat.
Further, most glue heads generate unsymmetrical glue patterns, due to action of the valve and the nozzle geometry, sometimes depositing a blob of glue trailing into a thin line, and sometimes ending a line with blob of glue.
Other disadvantages with typical prior glue heads have been high cost of the glue head, limited nozzle velocity, limited life, excessive comp times and unequal "ON" and "OFF" comp times, tendency of glue to build up on the nozzle, cleanability of the glue head and serviceability of the glue head.
Still further, prior glue heads have been bulky and sometimes too large for certain applications, such as needed in folding machines. The height of the glue head has tended to be a problem, primarily because the pressurized glue is normally fed vertically down into the glue head, the nozzle being at the bottom of the glue head, thus defining a tall assembly.